Semiconductor devices such as ICs, LSIs, etc, have heretofore been produced by repeating several times a series of processes comprising photolithography using photoresist compositions, etching, diffusion of impurities and wiring. Concretely, in the photolithographic process, a thin film of a photoresist composition is formed on a silicon wafer by means of, for example, spin coating, the resultant is exposed to active rays such as UV rays, etc., via a mask pattern for a semiconductor device, and then developed to give a resist pattern, and thereafter the silicon wafer thus having thereon the resist pattern acting as a protective film is etched. As the photoresist composition preferably used in the photo-lithography, there has been known a positive photoresist composition comprising, as basic components, an alkali-soluble novolak resin and a quinonediazido group-containing compound for which a resolution on the order of sub-microns(1 .mu.m or less) or still half-microns(0.5 .mu.m or less) is required and which uses UV rays such as g-line(436 nm) and i-line(365 nm) for exposure.
Recently, larger scale integrations in semiconductor devices are desired increasingly, requiring ultra-fine patterning on the order of quarter microns(0.25 .mu.m or less) in the mass production of ultra-LSIs, etc. However, the conventional positive photoresist composition comprising, as basic components, an alkali-soluble novolak resin and a quinonediazido group-containing compound hardly gives such an ultra-fine patterning as mentioned above. Therefore, it has been demanded to develop a resist using deep-UV rays of shorter wavelength (200-300 nm), excimer laser beams such as KrF laser and ArF laser, electron beams and X-ray. At present, a chemically-amplifying resists which can achieve a high resolution, use the catalytic and chain reactions of the acid to be generated by exposure to radiations, have a quantum yield of 1 or more, and can achieve a high sensitivity, have been of interest and developed intensively.
As one example of such chemically-amplifying positive resists, there is known a resist comprising a resin component derived from polyhydroxystyrene by substituting its hydroxyl groups by tert-butoxycarbonyloxy groups or the like and an acid-generating agent of an onium salt or the like (U.S. Pat. No. 4,491,628).
However, the above-mentioned, known chemically-amplifying positive resist was not satisfactory in practical use, since the resolution and the width characteristic in focus depth are not satisfactory, and since it may cause a problem so-called bridging that the crosssectional profile of the patterns to be made of the resist is often broadened upward like eaves. Concretely, when the chemically-amplifying positive resist coated on a substrate are exposed, stored for a while and then developed to give patterns, the patterns cannot have good profiles since the acids generated by the exposure are inactivated while the exposed resist films are stored. (which phenomenon is referred to as decrease in "post-exposure storage stability" of the positive resist, here-inafter)
The problem of post-exposure storage stability is peculiar to chemically-amplifying positive resists. When the bridging occurs, a desired wiring pattern cannot be given, which is a serious problem for the production of semiconductor devices. For the purpose of improving the post-exposure storage stability, there has been proposed a method in which a top coating layer is provided on a resist layer, whereby the inactivation of the acids generated by the exposure is prevented. In this method, however, the production steps are increased, which leads the decrease in throughput and high production cost as well. From these reasons, this method is unfavorable. Accordingly, there has been strongly demanded to develop a resist having good post-exposure storage stability without providing a top coating layer.
The above-mentioned chemically-amplifying positive resist has another problem that it characteristically depended on substrates to which they are applied, and some of them, when applied on an unsuitable substrate such as a substrate coated with an insulating film such as silicon nitride (SiN), boron-phosphorus-silicate glass (BPSG) or the like film or on a substrate coated with titanium nitride (TiN), often formed resist patterns with poor profiles expanding downward to the substrates (which is referred to as "substrate dependency", hereinafter). It has been assumed that the acids generated by the exposure are inactivated by the action of amines remaining around the substrate in the film formation step, which causes the formation of resist patterns with poor profiles expanding downward to the substrates.
Moreover, when such resist is coated on a substrate coated with a metallic film such as an aluminum-silicon-copper (Al--Si--Cu) alloy film and a tungsten (W) film, the resist pattern formed is influenced by standing waves and the crosssectional profile of the resist pattern is waved. For the purpose of solving the problems of such substrate dependency and influence by standing waves, there has been proposed a method in which an anti-reflection coating layer is provided between the substrate and the resist layer. In this method, however, the production steps are increased, which leads the decrease in throughput and high production cost as well. From these reasons, this method is also unfavorable. Accordingly, there has been strongly demanded to develop a resist without an anti-reflection coating layer which can give resist patterns with good profile not depending on substrates to which it is applied and hardly influenced by standing waves.
In addition to the above-mentioned problems, the conventional resist compositions have still another problem. That is, when the resist compositions are prepared in solutions, they have such a poor storage stability that the solutions often generate solid substances while the solutions are stored. Accordingly, there has also been demanded to develop a resist composition capable of giving a resist solution which has a good storage stability and does not generate any solid substances during it is stored.
In these situations, we, the present inventors have assiduously studied so as to develop chemically-amplifying positive resist compositions free from the above-mentioned problems and, as a result, have found that by using, as a resin component whose solubility in an alkaline aqueous solution is increased by the action of acids, a mixture of two different polyhydroxystyrenes where the hydroxyl groups have been substituted by two different kinds of substituents in certain degrees, respectively, and also a mixture of the preceeding two different polyhydroxystyrenes and an organic carboxylic acid, it becomes possible to provide a chemically-amplifying positive resist composition sensitive to radiations such as UV rays, deep-UV rays, excimer laser beams (such as KrF laser, ArF laser, etc.), X-rays, electron beams, etc., which has a high sensitivity, a high resolution, high heat resistance, good width characteristic in focus depth and good post-exposure storage stability, has good storage stability when it is prepared in a resist solution, and gives resist patterns with good profiles on substrates without depending on substrates to which it is applied. On the basis of these findings, we have completed the present invention.